Enhancement: allow setting game executable

[cam1170]

As I don't think setting an executable for a game imported by itch is currently possible because that checkbox is controlled by the itch plugin which currently doesn't support installation/play support, it would be nice to somehow make this an option.

[SirPrimalform]

Hmm, the plugin doesn't seem to be correctly reading installed games for me. Is that function contingent on the itch client being installed in the default location? Neither the client nor the game library folder are in the default places on my machine.
If the locations are hardcoded, could I maybe fix it by editing a line in a .py file?

[tauqua]

This has been in the repo since #49 in August but only made it into a release in v0.0.6. I believe it should work regardless of the Itch client's location, as long as it's installed.